Communications

6 - COOH-modified tip. on NH,-region

- 4 - Y 2 Q) 2 - 2 0 .

0 -

-2

toward the surface

tip moves away from the surface -

1 1 . 1 , . , , I

0 100 200 300 400

Distance [nm]

8 I ' I ' I ' I ' I

6 - on CH,-region COOH-modified tip.

-? 4 - = ! -

2 -

toward the surface m T 2 - 9 0 - away from the surface

-2 1 0 100 200 300 400

Distance [nm] Fig. 5. a) Force-distance curve taken on an NH, region. b) Force-distance curve taken on a CH, region. Both of the measurements (a and h) were made with the same cantilever; although the absolute scale is arbitrary, the relative size of both curves is correctly represented.

tion at different pH-values demonstrate that the same fea- tures can be obtained compared to experiments in air.

To conclude, we have provided confirmation that chemical modification of the SFM tip can be used for chemical imaging. The preparation of the chemical FM-sensor can be achieved by chemisorbing molecules with functional groups on the tip. The attached functional groups determine the chemical interaction of the tip with the substrate surface. A wide range of these groups with different chemical properties can be attached leading to a large flexibility in choosing the proper tip. Work following these lines is underway in our labora- tory.

Making use of the specific tip-sample interaction, an identification and selection of the specific chemical sub- stances even on a molecular scale can be achieved. The limits of the lateral resolution are expected to be comparable to the resolution of a conventional SFM. This methodology might be promising for molecular identification in chemistry, biol- ogy but also for applications on technical surfaces, especially for systems where there would be little or no contrast by standard SFM.

Received: January 12, 1995 Final vesion: March 2, 1995

[I] G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Phys. Rev. Lett. 1982, 4Y,

[2] J. Frommer, Angew. Chem. Znt. Ed. Engl. 1992,3f, 1298 -1328. [3] U. Dammer (Ed.), Scanning 1993, 15, 257-264. [4] J. A. Stroscio, P. M. Feenstra, A. P. Fein, J . Vuc. Sri. Techno/. 1987, A5,

X38-341. [5] S. Akari, K. Friemelt, K. Glockler, M. C. Lux-Steiner, E. Bucher, K.

Dransfeld, Appl. Phys. 1993, A57, 221 -223. [6] C. M. Mate, G. M. McClelland, R. Erlandson, S. Chiang, Phys. Rev. Lett.

1987,59, 1942-1945. [7] L. A. Wenzler, T. Han. R. S. Bryner, T. P. Beehe, Rev. Sci. 1~7sfvum. 1994,

65, 85 -88. [XI S. Akari, E. W. van der Vegte, P. C. M. Grim, G. F. Belder, V. Koustos, G.

ten Brinke, G. Hadziioannou, App/ . Phys. Leu. 1994, 65, 1915-1917. [9] L. F. Chi, M. Anders. H. Fuchs, R. R. Johnston, H. Ringsdorf, Science

1993, 25Y, 213-216. [lo] N. A. Burnham, R. J. Colton, J. Vuc. Sci. Techno/. 1989, A7, 2906-2913. [Il l E. -L. Florin, V. T. Moy, H. E. Gauh, Science 1994, 264, 415-417. [I21 C. D. Frisbie, L. F. Rozsnyai, A. Noy. M. Wrighton, C. M. Lieher, Science

[I31 R. G. Nuzzo, A. L. Allara, J. Am. Chem. Soc. 1983, 105,4481-4483. [I41 H. Keller, W. Schrepp, H. Fuchs, T/7in SolidFilms 1992, Z i O j Z i i , 799-802. [I51 R. Singhvi, A. Kumar, G. P. Lopez, G. N. Stephanopoulos, D. I. C .

Wang, G. M. Whitesides, D. E. Ingher, Science 1994, 264, 696--698. J. L. Wilbur, A. Kumar, E. Kim, G. M. Whitesides, Adv. Muter. 1994, 6 , 600- 604.

[I 61 The commercial electronics used for our experiments (TMX 2000) cannot he used for the recording of force- distance curves with the 0.3 N/m cantilever as the preamplifier is driven into saturation. The poor sensitivity of the rigid cantilever might he a reason for the absence of hysteresis in Fig. 5b.

57 - 59.

1994,265,2071 -2074.

Efficient Two Layer LEDs on a Polymer Blend Basis**

By Jiirn Pomrnerehne, Horst Vestweber, Werner G u n , Ruiner E: Muhrt, Heinz Bassler,* Michael Porsch and Jorg Daub

There is much current interest in the development of effi- cient organic light emitting diodes (LEDs) with tunable emis- sion color.[' - 31 Our approach has been based upon the use of polymer blends as the active component, advantages be- ing their processibility combined with a broad range of avail- able chrornophore~.[~] Recently, we reported LEDs fabricated with poly-phenylphenylenevinylene (PPPV) and tristilben- amine (TSA) derivatives doped into various polymeric hosts. It turned out that incorporating, e.g. PPPV, into a polymeric binder enhances the quantum efficiency c~nsiderably.[~] Al- though not directly participating in charge transport a polar matrix can have a positive effect on LED performance by giving rise to an enhancement of the cell current via a reduc- tion of the energy barrier for charge injection.r61

[*I Prof. H. Bassler, Dr. J. Pommerehne, Dr. H. Vestweber, Dr. W Guss, Dr. R. F. Mahrt Fachhereiche Physikalische Chemie und Zentrum fur Materialwissenschaften Philipps-Universitit Marhurg Postfach, D-35032 Marhurg (Germany) Dr. M. Porsch, Prof. J. Daub Institut fur Organische Chemie, Universitat Regenshurg D-93053 Regenshurg (Germany)

L**] We are indebted to Prof. W. Heitz, Dr. A. Greiner, and R. Sander for material synthesis and continuous advice. This work has been supported by the Bundesministerium fur Forschung und Technologie.

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It has been ~ h o w n [ ~ - ~ ] that insertion of a cathodic block- ing layer for hole discharge raises the LED efficiency. The fabrication of two-layer structures is easy if the bottom layer of the device is insoluble as is the standard precursor PPV. Otherwise deposition of the second layer may cause material interpenetration. In this communication we report on the successful preparation of efficient two-layer polymer-blend LED structures produced using the spin coating technique and a combination of mutually incompatible solvents["] for the materials under consideration.

A circa 100 nm thick layer of approximately 20 YO tristilbe- neamine or a derivative thereof dissolved in polysulfone (PSu) was deposited onto a commercial IT0 layer ("Bal- tacron") via spin coating using chloroform as solvent (the chemical structures of the active materials are shown in Fig. 1). Subsequently, a second layer, also about 100 nm

Tn(stilbene)amine, TSA

Tn( 4-methoxystilbene)amine, MSA

2-(4-hiphenyl)-5-(4-t-hutylphenyl)- 1.3,4-oxadiazol, PBD

I 0 I

Polysulfone, PSu

Fig. 1. Chemical structures of the compounds

thick, of 2-(4-biphenyl)-5-(4-t-butylphenyl)-l,3,4-oxadiazole (PBD) dispersed in polystyrene was deposited from cyclo- hexane solution. Due to the incornpatability of the polar polysulfone and the apolar cyclohexane no interfacial mix- ing occurs during the spin coating process. An aluminum cathode completed the sandwich structure which had an ac- tive area of 0.13 cm2.

The LEDs were characterized on the basis of theirj(E) characteristics, their emission spectra as well as the emission intensity as a function of cell current. To estimate the en-

ergetic location of charge transporting states the oxidation and reduction potentials of MSA and PBD were measured by cyclic voltammetry. These experiments were carried out using an Amel 5000 electrochemical system interfaced to a personal computer. A three-electrode configuration con- tained in an undivided cell consisting of a platinum disc as working electrode, a platinum plate as counter electrode and an AgCl coated silver wire as "pseudo" reference electrode. All potentials are referenced against ferrocene/ferrocenium (FOC) as internal standard. Acetonitrile (for the measurement of PBD) or dichloromethane (for TSA) served as solvents. Tetra-n-butylammoniumhexafluorophosphate (TBAHFP) was used as supporting electrolyte.

The oxidation and reduction potentials of PBD have been determined as + 1.49 V and - 2.41 V vs. FOC.["] Under the premise that the energy level of ferrocene/ferrocenium is 4.8 eV below the vacuum level,"21 the HOMO and LUMO levels must be located at c - 6.3 eV and c - 2.4 eV, respec- tively, below vacuum. The HOMO-LUMO gap is 3.9 eV, i.e. 0.2eV bigger than the energy of the onset of optical absorption (S,+S, 0-0 energy). One should notice that ow- ing to the improved ion stabilization in a polar solvent voltammetry underestimates the HOMO/LUMO gap of a chromophore embedded in a solid matrix below the glass transition temperature where dipolar relaxation is arrested. The data, therefore, confirms the motiv that the binding energy of an excited state relative to that of a radical anion/ cation pair E,,, > 0.2 eV in accord with previous work sug- gesting that E,,, be about 0.4 eV.[13]

The oxidation potential of TSA is 0.32 V, hence the HOMO should be at z - 5.1 eV below vacuum. The reduc- tion potential turned out to be beyond the instrumental limit (- 2.5 V), consistent with optical absorption that locates the S,+S, (0-0 transition) at 2.9 V. Adopting the above result that the voltammetrically determined HOMO/LUMO gap exceeds the optical gap by 0.2 eV one would expect the re- duction potential to be -2.8 V and, concomitantly the elec- tron transporting level to be located at - 2.0 eV below vacu- um. From the above value of E,, (TSA) one would expect the energy barrier for hole injection from I T 0 to be c0 .3 eV (assuming q (ITO) = 4.8 eV). This is close to the value x+ = 0.2-0.25 eV inferred from Fowler-Nordheim plots of j ( E ) curves for TSA-like systems in various polymeric binders.[61 Note that charge transporting levels of molecules embedded in random matrices are always inhomogeneously broadened, variation being typicaly 0.1 eV, implying that energy barriers for injection from a metallic or semimetallic electrode into tail states are lowered relative to an ordered counterpart structure.

The energy level structure for a TSAjPBD double layer that follows from cyclic voltammetry is shown in Figure 2. On this basis it can be predicted that the transport of holes from TSA to PBD is impeded by a 1.2 eV energy barrier, the barrier for (reverse) electron flow being c 0.4 eV only. Fur- thermore, electron injection from A1 (possibly through an interfacial A1,03 layer) should be facilitated, although holes

552 0 VCH Verlagsgerellschaft mbH, 0-69469 Wecnheim 1995 O935-9648/95/O606-0552 $5 O O i 2510 Adv Mater 1995, 7, NO 6

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- 2 . 0

LlJMO - 2 . 4

LlrMO

- 4 . 8 - -5.1 HOMO -6.3

HOMO

ADVANCED MATERIALS

- 4 . 2 -

El eV vacuum level 0

-5

-6

-7

PBDiPS Al Fig. 2. Energy level structure for an ITOITSA(PSu)lPBD(PS)IAl assembly as determined by cyclic voltammetry.

should remain the majority carriers. Hence, a space-charge layer should be established at the internal interface that re- duces the electric field at the anode. This is supported by comparing j (E ) curves for an ITOITSA(PSu)(Al and an ITO(TSA(PSu)PBDIAl device (Fig. 3). The latter is shifted towards higher fields equivalent to a reduction of the field at I T 0 by a factor of 0.8. In order to maintain a given voltage drop across the cell this implies an increase of the field inside the PBD layer by a factor of 1.2.

L I I 1

- 0.20 0.40 0.60 0.80

1 /E /* 1 0-6 crnV-' Fig. 3. j ( E ) curves for a single-layer LED and a double-layer LED plotted on a Ig j vs. E - scale.

A change of the field across the interface by AF = 0.4 F corresponds to an internal positive charge density (per unit area) as shown in Equation 1, where for F = lo6 Vcm-' and E = 3, N , = 6.6 x 10" charges cm-' is obtained.

Introducing an internal blockade against mono-molecular (non-radiative) loss of majority carriers by discharge at the cathode should increase the quantum efficiency of the LED.

In fact, Figure 4 bears out an increase of the light intensity of the double layer by up to three orders of magnitude as compared to single-layer devices. On an absolute scale the quantum efficiency achieved with a TSA(PSu)/PBD cell was about 1.3 %, which to the best of our knowledge is the highest efficiency of an LED operating with aluminum as cathode material.

1 o - ~

. ?

3

0

\

.- u)

v TSA/PSU 7 TSA/PBD 0 MSA/PBD

I O - ~ 1 0 - ~ lo -*

current/ A

Fig. 4. Relative electroluminescence intensities as a function of the cell current for various LEDs. The results for the TSAjPBD system are equivalent to an external quantum yield of 1.3%.

The enhancement of the LED quantum efficiency by a blocking layer for majority carriers is readily understood in terms of carrier recombination kinetics. Unless injection as well as transport of holes and electrons is exactly balanced the probability that a minority carrier i.e. an electron, in the present system, will recombine with a hole rather than get discharged at the anode is given by Equation2, where k,,, = yn, is the rate constant for recombination, n, the volume concentration of holes and y the (bimolecular) re- combination coefficient.

Since z,, = d/pF is the carrier transit time, d the sample thickness and p the electron mobility, i.e. is a measure of the rate of monomolecular recombination of minority carri- ers, Equation 2 can be rewritten as shown in Equation 3.

Unless the cell current is space-charge limited (in which case n,,,,, = 3~~,,F/2ed['~'), pF/yn+ d >> 1. Since in the strong scattering limit, equivalent to Langevin recombina- tion yip = e/sE,, E being the dielectric constant, Equation 4 holds.

(4)

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In the absence of a carrier blocking layer, n + equals the stationary charge density n, =j/(epE).[l6I For d =

cmZ/Vs a n d j = lo- ' Acm-', n, = 6 x lo1' cm-3 and P,,,x0.03 is obtained. In the case of an interfacial space charge n + = N + d, N , being given by Equation 1 and pF/ yn, d = FlAFyielding P,,, % 0.3. It is obvious that build-up of an interfacial space charge increases the probability that an injected minority carrier will find a countercharge to re- combine with. Recombination occurs inside the TSA and MSA layers, respectively as evidenced by the electrolumines- cence spectra shown in Figure 5.

1 .o

. 0.8 ?

\ 0.6 0

ZI

m c .-

0.4 -w K .-

0.2

0.0

I 1 I I I

400 500 600 700

wavelength/ nm

Fig 5 Electroluminescence spectra of rSA/PBD and TSA/PSu

The observed increase of the LED efficiency by a PBD blocking layer exceeds the factor estimated on the above basis indicating that such a layer also has a favorable effect on other efficiency controlling factors. Since the reduction potential of PBD is zz 0.4 eV less than that of TSA electron injection is facilitated. Furthermore, the recombination zone is spatially isolated from the cathode thus eliminating excit- ed-state quenching by the metal.[7, ' ' 9 '*I

The above analysis, though crude, indicates that the LED efficiency should scale inversely with the mobility of the ma- jority carriers as long as the majority current is injection- rather than space-charge limited, because the stationary concentration of recombinations centers scales inversely with their transport velocity. This provides an explanation of why in single-layer cells the efficiency increases upon dilu- t i ~ n . [ ~ ] The concomitant decrease of the diffusivity of the generated excited state will also be of advantage because quenching by non-fluorescent impurities becomes less effi- c i ~ n t . [ ' ~ ]

Received: January 9, 1995 Final version: March 1, 1995

[ I ] C. W Tang, S. A . van Slyke, Appl. Phys. Lett. 1987, 5f, 913. [2] C. Adachi, T. Tsutsui, S. Saito, Jpn. J . Appl. Phys. 1989, 55, 1489. [3] P. L. Burn, A. B. Holmes, A. Kraft, D. D. C. Bradley, A. R. Brown, R. H.

[4] H. Vestweber, A. Greiner, U. Lemmer, R. F. Mahrt, R. Richert, W. Heitz, Friend, W. Symer, h'uture 1992, 356, 47.

H. Bissler, A d i ~ . M o m . 1992, 4, 661.

[5] H. Vestweber. J. Oberski, A. Greiner, W. Heitz, R. F. Mahrt, H. Bdssler, Adv. Matar. Opt. Electron. 1993, 2, 191.

[6] H. Vestweber, R. Sander, A. Greiner, R. F. Mahrt, H. Bassler, Synth. Met. 1994, 64, 141.

[7] A. R. Brown, J. H. Burroughes, N. Greenham, R. H. Friend, D. D. C. Bradley, P. L. Burn, A. Kraft, A. B. Holmes, Appl. Phys. Lett. 1992, 61, 2793.

[S] D. D. C. Bradley, Synth. Met. 1993, 54, 401. [9] C. Zhang, S. Hoger, K. Pakbaz, F. Wudl, A. J. Heeger, J . Electron. Mater.

1994, 23. 453. [lo] I. D. Parker, Q. Pei, M. Marrocco, Appl. Phys. Left . 1994, 65, 1272. [ l l ] To provide additional proof for the reversibility of the formation of the

radical ions investigations by UV/vis/NIR spectroelectrochemistry were undertaken. Both, the formation of the radical anion of PBD and the formation of the radical cation of TSA display sharp isosbestic points and absorption signals at 309, 517, and 1105 nm for PBD and 225,460,1115, 131 5 nm for TSA, . UV/vis/NIR spectroelectrochemical measurements were carried out with a Perkin Elmer Lambda 9 spectrophotometer. Po- tentials were adjusted by an AMEL 550 potentiostat. The construction of the thin-layer cell is described in J. Salbeck, I. Aurbach. J. Daub, Dechema Monographien, Vol. 112, p. 177, VCH, Weinheim 1988. J. Salbeck, Anal. Chem. 1993, 65, 2169.

[12] This estimate is calculated on the basis of a rather crude approximation neglecting solvent effects using the standard electrode potential (E') for the normal hydrogen electrode (NHE) at about -4.6 eV (A. J. Bard, L. R. Faulkner, Electrochemicul Methud~y - Fundamentals and Applications, Wi- ley, New York, p. 634) on the zero vacuum level scale and a value of 0.2 V vs. NHE for the potential of FOC (in acetonitrile, see: H.-M. Koepp, H. Wendt, H. Strehlow, 2. Electruchem. 1960, 64, 483.

[13] R. Kersting. IJ . Lemmer, M. Deussen, H. J. Bakker, R. F. Mahrt, H. Kurz, V. I. Arkhipov, H. Bassler, E. 0. Gobel, Phq's. Rev. Lelr. 1994, 73, 1440.

[14] W. Helfrich, in Physics and ChemistrjJ of the Orgunic Solid Stute. Vol. 111 (Eds: M. M. Labes, A. Weissberger) Interscience Publ., New York 1967, p.1.

[I51 M. Silver, M. Sharma, J . Clzem. Phys. 1967, 46. 692. [I61 A. R. Brown, N. C. Greenham, J. H. Burroughes, D. D. C. Bradley, R. H.

Friend, P. L. Burn, A. Krdft, A. B. Holmes, Chem. Phjs. Lett. 1992,200, 46.

[17] K. H. Drexhage, J. Lumin. 1970, f/2, 693. [IS] H. Kruczewskd, H. Bassler, 1 Lumin. 1977, /5, 261. [I91 U. Lemmer, R. F. Mahrt, Y Wada, A. Greiner, H. Bassler, E. 0. Gobel,

Appl. Phys. Lett. 1993, 62, 2728.

Rectifying Properties and Photoconductivity of Tetraruthenated Nickel Porphyrin Films"" By Koiti Araki, Lucio Angnes, and Henrique E. Toma*

Porphyrin films constitute an exciting area of research because of their electronic, photochemical and chemical properties. Recently, Malinski and Taha,"] successfully em- ployed carbon fiber microelectrodes modified with the poly- tetrakis(3-methoxy-4-hydroxyphenyl)porphynate nickel(@ complex to monitor the NO produced by a single cell.

Here, we report the properties and characterization of films of the polymetallated porphyrin (I) obtained by attach- ing four Ru(bpy),CIQ complexes (bpy = 2,2'-bipyridine) to the peripheral pyridyl residues of the meso-tetra(4-pyr- idy1)porphynate nickel(@ complex (NiTPyP) . This modified porphyrin has been isolated as [NiTPyP{ Ru(bpy),CI},- (TFMS), (TFMS = trifluoromethanesulfonate anion). It

[*I H. E. Toma, Prof. K. Araki, L. Angnes Inbtituto de Quimica, Universidade de Sao Paulo Caixa Postal 26077, CEP 05599-970, Sao Paulo, SP (Brazil)

knowledged. [**I Financial support from PADCT, CNPq and FAPESP is gratefully ac-

554 c) VCH Verlayygesellschuft mhH D-6Y46Y Wemherm, f9Y5 OY35-Y64B/Y5/O606-0554 $ 5 O0+ 25/0 Adv Mater 1995, 7. No 6